The present invention relates generally to the conveying of signals from a device implanted beneath the surface of the skin to a second device located external to skin. The invention may be used, for example, to convey information from an implantable hearing aid to an external readout device.
The use of implantable devices requires a means for control and programming of the device characteristics from an external signal. In the case of a hearing aid, it may be necessary to set the gain, maximum power output and frequency response after implantation, to provide an optimal setting of parameters for the patient. In this regard, it would be advantageous to provide for the readout of data and parameters from the implanted device.
Control of an implanted device by an external signal can be done by transmitting a modulated carrier signal from an external coil and providing for an internal coil to receive and demodulate the signal. Because the transmitted signal in this case is generated externally, relatively large power sources can be used. Although it is possible to reverse this process to transmit from the implanted device to the external circuitry, it is generally not practical to do so because of the limited power available within the implanted power source.
The present invention provides a solution to this problem in the form of a method and apparatus for transmitting a signal from the implanted device to the external device without requiring an internal transmitter and without requiring excessive power to be consumed by the implanted circuitry. In accordance with the inventive method a signal is conveyed from the implanted device to the external device by modulating the impedance of the implanted device in accordance with the signal to be conveyed and then inductively supplying energy to the implanted device via the external device. By monitoring the energy delivered, the external device ascertains the modulated impedance of the internal device and thereby ascertains the signal being conveyed.
In the presently preferred embodiment the external device employs a class C power amplifier coupled to an external coil. The external coil is placed in close proximity to an implanted coil forming part of the implanted device, thereby defining primary and secondary windings of a transformer. The internal device includes a rectification circuit which can be switched between modes of half wave and full wave rectification in response to the signal to be conveyed. The class C power amplifier delivers power through the transformer defined by the external and internal coils to the implanted rectifier circuit. A current sensing resistor in the power supply of the power amplifier monitors the energy delivered to the rectifier, specifically monitoring the current pulses drawn by the power amplifier. A decoder circuit responsive to the current sensing resistor monitors the frequency of pulses delivered by the class C amplifier and thereby monitors the rectification modes to thereby ascertain the signal being conveyed by the implanted device. If desired, the energy delivered to the rectifier circuit may be used to provide operating power to the implanted device or to charge a suitable energy storage means.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.